In semiconductor device fabrication, material is deposited and, oftentimes, patterned to form the many integral layers in a semiconductor device and integrated circuit (IC). Patterning involves masking certain material that is not to be removed and removing unmasked portions of certain material. Oftentimes, photoresist or silicon nitride are used for the masking material. It is important to be able to selectively control the material that is etched in order to not remove the masking material while removing unmasked portions of the material underlying the mask.
Furthermore, oftentimes it is desirable to selectively remove unmasked portions of various materials at different rates. For example, it is often desirable to etch silicon dioxide (SiO.sub.2) in the presence of borophosphosilicate glass (BPSG), without simultaneously removing unmasked BPSG. It is also desirable to etch SiO.sub.2 without simultaneously etching silicon (Si).
For example, BPSG is used as a dielectric layer because it is easily deposited, provides some margin against mobile ion contamination, and is easily planarized with CMP. However, in this application, one needs to clean or etch SiO.sub.2 layers in the presence of BPSG. It is, therefore, necessary to have a method of etching SiO.sub.2 at a rate equal to or faster than the etch rate of BPSG.
Hydrofluoric acid (HF) is a conventional component of oxide etchants. Certain HF solutions etch thermally grown SiO.sub.2 at a rate of about 1,200 .ANG./minute. Deposited SiO.sub.2, in general, tends to etch at a faster rate than thermally grown SiO.sub.2. HF solutions provide selective etch rates of about 100:1 or better for SiO.sub.2 :Si. HF, however, etches BPSG at a rate of about ten times the rate it etches SiO.sub.2, whether it is thermally grown or deposited. This is typically due to the dopants (e.g., phosphorus and boron) in BPSG. This makes it extremely difficult to selectively etch SiO.sub.2, particularly thermally grown SiO.sub.2, in the presence of BPSG.
Dilute HF and buffered oxide etchant (BOE), which is a dilute mixture of HF and ammonium fluoride (NH.sub.4 F), usually in a ratio of about 1 part HF to 6 parts NH.sub.4 F (by volume), are the most prevalent conventional SiO.sub.2 etchants. Both dilute HF and BOE are typically diluted in water in a ratio of about 10:1, 20:1, and 100:1 (i.e., 10, 20, or 100 parts (by volume) of water to one part reactive etchant components). Another conventional etchant, sold by Olin Hunt in Chandler, Ariz. that has been used to etch oxide is referred to as "SUPER Q," which is a mixture of NH.sub.4 F and phosphoric acid (H.sub.3 PO.sub.4).
Using such conventional etchants, the highest ratio of reported selectivity for etching SiO.sub.2 in the presence of BPSG is 1:2, corresponding to their respective etching rates. Thus, it is desirable to provide an improved SiO.sub.2 etchant with enhanced selectivity for etching SiO.sub.2 in the presence of BPSG.